An example of a conventional light-emitting device is described in U.S. Pat. No. 9,525,148 (Kazlas et al., issued Dec. 20, 2016). FIG. 1 is a drawing depicting an exemplary representation of such a light-emitting device. As a structural overview, a light-emitting device includes an anode 104 and cathode 100, and a light-emitting or emissive layer 102 containing a material that emits light 107. Within the light-emitting emissive layer 102, light is produced upon electron and hole recombination to generate the light 108. The emissive layer 102 may be an inorganic or organic semiconductor layer, or a layer of quantum dots (QDs). At least one hole transport layer 103 is located between the anode 104 and the emissive layer 102, which provides transport of holes from the anode and injection of holes into the emissive layer. Similarly, at least one electron transport layer 101 is located between the cathode 100 and emissive layer 102, which provides transport of electrons from the cathode and injection of electrons into the emissive layer.
In such structures, the layer (or layers) 101 between the cathode 100 and emissive layer 102 is termed the electron transport layer (ETL), and the layer (or layers) 103 between the anode 104 and the emissive layer 102 is termed the hole transport layer (HTL). The ETL and HTL are collectively referred to more generally as charge transport layers (CTL). The purpose of these CTLs is to provide an ohmic contact to the respective electrode (anode or cathode), and to provide energetic alignment for injecting carriers (electrons or holes) into the emissive layer. To enhance the luminescence quantum efficiency of a QD LED, the mobility of the charge carriers should ideally be as high as possible in absolute terms, to reduce resistive losses, and be balanced in relative terms between electrons and holes, to reduce non-radiative auger recombination. These criteria should be met both within each layer, including each HTL, ETL, and emissive layer, and also between layers, such as balancing HTL hole mobility and ETL electron mobility.
Several approaches, therefore, have been explored to enhance the luminescence quantum efficiency of the light-emitting device. One approach is balancing injection of electrons from the ETL into the QDs in the emissive layer with the injection of holes from the HTL by adjusting the relative mobilities of electrons and holes, such as by employing injection barriers. Examples of such an approach include particular material selection as taught in KR 101626525 (Yang et al., issued Jun. 1, 2016) and CN 106410051 (Zheng et al., published Feb. 15, 2017), or by using an interlayer to adjust the injection barrier as taught in PMMA: DOI: 10.1038/nature13829. Another approach is increasing the mobility of both charge carriers in the QD layer by substituting long ligands with shorter ligands or inorganic passivating groups, as taught in DOI: 10.1038/s41566-018-0105-8. Another approach is increasing the mobility of the limiting charge carrier in the emissive layer, typically the holes as they have larger mass than the electrons, by mixing the QDs with, for example, a hole transporting material, as taught in DOI: 10.1016/j.cap.2016.12.024 and U.S. Pat. No. 8,343,636 (Kwan-Yue et al, issued Jan. 1, 2013).
As is termed by those of ordinary skill in the art, a Type I QD is a QD in which each electron and hole is confined to the core of the quantum dot. When both the electron and hole are confined to the core of the quantum dot, the wavefunctions of the electron and hole have maximum overlap within the QD core, which provides for a high rate of recombination in the emissive layer. Accordingly, the conventional teaching is to employ Type I QDs as the optimum QD to use for light-emitting devices (see, e.g., DOI: 10.1126/science.aac5523) because they have the highest core overlap between electron and hole wavefunctions, and therefore most encourage radiative recombination. However, films made of colloidal Type I QDs exhibit very low charge carrier mobilities, and in particular, often a very low hole mobility. This can result in recombination in only a single monolayer of QDs adjacent to the HTL, resulting in overall low quantum efficiency devices which operate at high voltage. In this manner, the high recombination rate of Type I QDs can be offset by the lower carrier mobility (especially low hole mobility), which reduces the resultant luminescence quantum efficiency of a QD LED.